水热碳化制备榴莲壳复合焦及其电化学性能

董向元，张恒瑞，陈 祥，李 硕，郭淑青

水热碳化制备榴莲壳复合焦及其电化学性能

董向元，张恒瑞，陈 祥，李 硕，郭淑青※

（南京工程学院能源与动力工程学院，南京 211167）

为研究水热碳化处理对榴莲壳复合改性焦性能的影响，将榴莲壳原料及250 ℃、10 h制备的水热焦分别与层状氢氧化镁铝（Mg/Al Layered Double Hydroxide，MgAl-LDH）复合，获得榴莲壳与MgAl-LDH复合焦MgAl-LDH@DP和榴莲壳水热焦与MgAl-LDH复合焦MgAl-LDH@HC，分析比较两种焦的特性以及电化学性能。结果表明，同MgAl-LDH@DP相比，MgAl-LDH@HC有更强的活性含氧官能团，对LDH纳米片有较好的分散性。MgAl-LDH@DP焦表面有大量针状结构，而MgAl-LDH@HC呈不规则片状结构，表面疏松多孔，BET（Brunauer-Emmett-Teller）比表面积为62.96 m2/g，平均孔径14.81 nm，BJH（Barrett-Joyner-Halenda）累积吸附孔容积为0.24 cm3/g，均高于前者，更有利于电荷储存和电子传输。在KOH溶液为电解质、复合焦为工作电极的三电极系统中，循环伏安曲线和恒电流充放电曲线分别接近矩形和三角形，同MgAl-LDH@DP相比，MgAl-LDH@HC有较好的电容特性和倍率性能，低频时交流阻抗曲线斜率更大，离子扩散阻力相对较小，有潜力作为超级电容器电极材料应用。

生物炭；水热焦；电化学性能；榴莲壳；层状双氢氧化物

0 引 言

水热碳化处理是利用水在亚临界时扩散性较强、溶解性较好的特殊性质，使生物质组分快速发生脱水、脱羧和芳构化等一系列化学反应[1-3]，形成表面富含活性氧基团、具有一定孔隙结构且含碳量较高的水热焦[4-8]。因其特殊的性质，水热焦在燃烧[9-10]、污水处理[11-12]和储能[13-14]等领域的应用引起了研究者的广泛关注。

而水热焦作为电极材料应用时[15-20]，为进一步提高其比表面积，改善其孔隙结构，从而提高其电荷存储能力，一般采取将水热焦与KOH等碱性物质混合，再经高温煅烧进行活化[19-20]，这在一定程度上增加了工艺复杂性和制备成本。

层状双金属氢氧化物（Layered Double Hydroxide，LDH）是含有两种或两种以上金属的无机功能材料，也可在较温和的条件下，用水热法合成。其具有原料易得、成本低廉、活性位点均匀分散等优点，近年来成为了超级电容器电极材料的研究热点[21-22]。但研究发现，LDH稳定性不理想[22]，急需利用其结构易于调整，并易于复合其他材料的特点，进行改性。

将水热碳化法获得的水热焦与LDH复合，有望利用LDH材料的优势，改善水热焦的电容特性；利用水热焦的结构特点，改善LDH的稳定性，共同利用水热法合成的优势，实现电极材料性能的改进。相关研究已引起了部分研究者的关注[22-27]。Zhang等[22]将柚子皮水热焦与CoNiAl-LDH复合，探索了复合焦的电容特性，发现其具有较高的比电容和良好的循环稳定性。Lai等[26]以细菌纤维素水热焦为模板进行N掺杂后，同NiCo-LDH复合，制取的复合焦展现了良好的电化学性能。可见，将水热焦与LDH复合的研究思路是可行的，且复合焦表现出了作为电极材料优良的电容性能。

许多研究者针对不同来源生物质水热碳化做了大量研究，发现果壳和果皮类废物水热焦孔隙相对较好[28-30]，但对于壳占果质量约70%的榴莲壳研究相对较少，作者所在课题组研究了榴莲壳水热焦的特性和电化学性能[31]，并探索了氢氧化钾催化条件下制备的榴莲壳水热焦与LDH复合焦的特性，但榴莲壳及其在纯水环境下制备的水热焦与LDH复合焦特性的具体研究亟待开展，相关研究对于理解水热碳化处理对复合焦结构调整和性能影响有重要意义。

基于此，本研究以榴莲壳为原料，在纯水溶剂中，利用水热碳化法将其制备成水热焦，再将水热焦与MgAl-LDH再次利用水热法进行复合，获得复合焦，为分析水热碳化处理对复合焦特性的影响，同时将未处理的榴莲壳与MgAl-LDH复合，研究两种复合焦的特性和电化学性能，以期为研究水热碳化处理对生物质水热焦复合改性的影响提供参考。

1 材料与方法

1.1 试验材料和方法

试验所用物料榴莲壳采自南京市水果超市，清洗去除浮灰后晾干，破碎至粉末状，粒径不超过2 mm，其干基C质量分数为41.22%。试验中用水均为去离子水，所用化学试剂均为分析纯，订购于上海阿拉丁试剂有限公司。

榴莲壳水热碳化试验及与层状双金属氢氧化物复合试验均在316 L反应釜中进行。

榴莲壳水热碳化处理具体过程为：将榴莲壳与去离子水按质量比1∶10充分混合，放入釜中，密闭加热，为使榴莲壳水热碳化反应充分，结合前期研究结果，选择反应温度为250 ℃，停留时间为10 h，试验结束，通入冷却水，冷却至室温和环境压力后取出物料，过滤分离干燥，获得榴莲壳水热焦记为HC（Hydrochar）。

榴莲壳和榴莲壳水热焦分别与MgAl-LDH复合的试验过程为：取固体Mg(NO3)2·6H2O和Al(NO3)3·9H2O，以Mg∶Al摩尔比为3∶1的比例加入至去离子水中溶解，取适量溶液，向其中以溶液与固体质量比10∶1加入HC，搅拌均匀，随后滴加NaOH与Na2CO3溶液，调理混合溶液pH值为10～11，将其放入反应釜中，为使MgAl-LDH成功复合，结合文献研究[22]，选择反应温度为180 ℃，时间为10 h，反应结束，冷却、过滤、干燥获得MgAl-LDH@HC。为探索水热碳化处理对复合焦特性的影响，以未经处理的榴莲壳（Durian shell，DP）与MgAl-LDH复合作为参照，复合试验过程同上，获得的复合焦记为MgAl-LDH@DP。

1.2 分析方法

微晶结构采用粉末X射线衍射仪（X-Ray Diffraction，XRD）分析，Cu 靶辐射，间隔为0.02°；微观形貌和表面官能团分别采用扫描电子显微镜（Scanning Electron Microscopy，SEM）和傅里叶变换红外光谱（Fourier Transform Infrared Spectrometry，FTIR）分析；比表面积采用N2吸附和脱附等温线进行分析；表面元素组成采用X射线光电子能谱（X-ray Photoelectron Spectroscopy，XPS）进行测试。

为分析复合焦的电化学性能，将复合焦制备成工作电极，具体制备方法参见文献[15]。在三电极体系下，采用CHI660E电化学工作站进行测试，具体条件为：以Hg/HgO作为参比电极，铂片为对电极，自制电极为工作电极，KOH溶液（6 mol/L）为电解质，分别进行循环伏安、恒电流充放电和电化学阻抗谱测试。

质量比电容g依据恒电流充放电曲线计算，如公式（1）

依据黄观音在龙州县的生长特性，通过多年加工试验，总结出制作花香型黄观音的新型制茶方法：黄观音秋季鲜叶→轻晒青(地表温度28 ℃，空气湿度64%，30 min)→轻摇青(1 min，2次)→室内萎凋(空调控温)→揉捻(40～60 min)→发酵(4～5 h，控温控湿)→理条(针型)→烘干→提香→成品茶。

2 结果与分析

2.1 复合焦结构特性

榴莲壳经水热碳化处理后，其纤维素晶体结构受到破坏，发生了降解碳化，其干基碳质量分数为70.29%，与MgAl-LDH复合后获得MgAl-LDH@HC，主要在衍射角11.23°、22.64°、33.98°和60.11°出现了（003）、（006）、（012）和（110）特征峰（图1），这归因于类水滑石LDH的相平面，说明MgAl-LDH成功复合在榴莲壳水热焦表面，层状双金属氢氧化物独特的类水滑石结构也有利于离子在多层空间的快速扩散。而MgAl-LDH@DP是榴莲壳原料与MgAl-LDH直接水热复合而获得，在衍射角22.37°和34.39°处可见较强的纤维素晶体结构特征峰，与LDH特征峰同时存在。从XRD谱图中可以看出，榴莲壳原料及其水热焦都成功成为了复合焦的活性组分，并分散了LDH纳米片。

为进一步分析MgAl-LDH在榴莲壳及其水热焦上的生长情况，图2给出了复合焦的FTIR谱图。从图中可以看出，MgAl-LDH@HC和MgAl-LDH@DP在3 429、2 925、1 622 cm-1处均出现了吸收峰，强度略有差异，分别由-OH，-CH，C=O或C=C振动引起，而且在1 383和648 cm-1均出现了硝酸根和Al-O的振动吸收峰。可见，两种复合焦均有丰富的含氧官能团，因其带有负电荷，容易使金属离子扩散进入榴莲壳水热焦HC或榴莲壳DP中，使得MgAl-LDH原位复合在HC或DP表面。

两种复合焦红外吸收峰明显的不同之处在于，MgAl-LDH@DP在1 053 cm-1处存在较强的C-O-C吸收峰，而MgAl-LDH@HC只在1 111 cm-1出现非常弱的吸收峰，这主要是榴莲壳经水热碳化处理后，半纤维素等糖苷键降解断裂，并发生了脱氧反应所致，但同时榴莲壳组分也发生了聚合和芳香化反应，HC碳质量分数达70.29%，故1 622 cm-1处聚合物特征峰强，其可增加复合焦的活性和亲水性。

MgAl-LDH与HC复合时，主要以榴莲壳水热焦HC作为碳源和框架，Mg和Al金属离子分布在HC表面，因此MgAl-LDH@HC复合焦表面元素以C、O为主，如图3a，C和O原子百分比分别为74.11%和22.40%，而Mg和Al原子百分比分别为2.42%和1.06%。在结合能284.80和532.14 eV处可见C 1s和O 1s强峰，而在50.31和74.71 eV处出现Mg 2p和Al 2p弱峰，再次证实Mg和Al金属离子已成功复合在水热焦表面。C 1s和O 1s的扫描谱解析如图3b和3c，C 1s谱图在结合能284.53、285.73、288.18 eV处出现3个特征峰，分别对应C=C、C=O化学键，其中C=O在碱性电解液中具有电化学活性，可提供主要的赝电容；O 1s谱图在531.08、531.93、532.78 eV处存在3个明显的峰，分别对应Al2O3、-OH和-O-、C=O化学键，其均可增加复合焦的亲水性，为复合焦的润湿性及其在电极溶液中的活性提供保证。

为深入分析复合焦的孔隙结构，图4给出了两种复合焦的N2吸附和脱附等温曲线。可以看出，在相对压力/0较高时，两种复合焦的吸附等温线接近Ⅳ类型，脱附均有回滞，而MgAl-LDH@DP脱附回滞更加明显，这主要是两种复合焦孔隙结构不同所致。MgAl-LDH@HC复合焦BET比表面积为62.96 m2/g，BJH累积吸附介孔容积为0.24 cm3/g，HK微孔容积为0.03 cm3/g，平均孔径为14.81 nm。而MgAl-LDH@DP复合焦BET比表面积和BJH累积吸附介孔容积分别为38.37 m2/g、0.11 cm3/g，HK微孔容积为0.02 cm3/g，平均孔径为10.34 nm。可见，两种复合焦，介孔容积均较微孔容积高一个数量级，吸附以大孔和介孔为主，这有利于吸附质的传输和电荷的扩散与存储。且MgAl-LDH@HC孔隙更为丰富，其比表面积是MgAl-LDH@DP比表面积的1.64倍，介孔容积和平均孔径均较大。这主要是因为榴莲壳经水热碳化处理后，可溶性物质进入液相产物，并有少量CO2等气相产物生成[2]，使得固体产物水热焦形成了一定程度的孔隙结构，其与层状结构的MgAl-LDH复合，促进了核壳多孔结构的生长，复合焦孔隙得到了更好地发展，为吸附质和电荷传输提供了更宽的通道。

MgAl-LDH@HC和MgAl-LDH@DP微观形貌明显不同，如图5。图5a显示MgAl-LDH超薄的纳米片以片状堆叠的形式负载到HC焦表面，使得复合焦具有不规则片状结构，表面疏松多孔，有利于暴露电活性位点，可促进MgAl-LDH和OH-间氧化还原反应，通过榴莲壳水热焦加速超薄纳米片间的电子传输速率。而图5b显示MgAl-LDH@DP焦表面有大量针状结构，孔隙不如MgAl-LDH@HC丰富，所以比表面积较低，这与氮气吸脱附等温线分析结果相对应。

2.2 复合焦电化学特性

为研究复合焦在超级电容器中的应用，并确切了解复合焦的电容性能，依据参考文献[15]和[22]，选择了有参比电极的三电极系统，电解质选择为6 mol/L KOH。

在1 A/g的电流密度下，两种复合焦的恒电流充放电曲线均接近三角形（图6c），但在0.2～0.3 V存在过渡区域，这与循环伏安曲线中凸起的位置相一致，均是由杂原子所致。同MgAl-LDH@DP相比，MgAl-LDH@HC有较好的库伦效率。文献[22]在1～10 A/g电流密度下，研究了柚子皮水热焦与层状金属氢氧化物复合焦的恒电流充放电特性。课题组扩大电流密度测试范围，在1、5、10、20 A/g电流密度下，探索了榴莲壳水热焦的恒电流充放电规律[31]，发现榴莲壳水热焦均具有较好的倍率性能。为简化测试且不影响分析结果，本研究选取1、5、20 A/g的电流密度，研究MgAl-LDH@HC复合焦的倍率性能，从图6d中可以看出，当电流密度从1增至20 A/g时，MgAl-LDH@HC的恒电流充放电时间逐渐减小，这主要是，低的电流密度有利于MgAl-LDH@HC促进电解质与其活性位点接触，氧化还原反应充分且反应速率较高，同时MgAl-LDH@HC孔隙相对发达，为电子传输提供了保障，电子导电性较好。电流密度为1A/g时，MgAl-LDH@HC质量比电容为1 250 F/g，当电流密度增加至20A/g时，仍有约56.32%的电容保持率，说明MgAl-LDH纳米片在榴莲壳水热焦HC上分布较为均匀，有利于OH-离子的渗入。

两种复合焦的电化学阻抗谱如图6e，高频时，两种复合焦的电荷转移电阻和等效串联电阻相差不大，曲线基本重合；但在低频时，同MgAl-LDH@DP相比，MgAl-LDH@HC阻抗谱斜率稍陡，说明榴莲壳经水热碳化处理后，制得的复合焦扩散阻力较小，有利于离子扩散，电性能较好。

综合比较分析，水热碳化处理后制得的水热焦经改性后有良好的电容性能，更有潜力作为超级电容器电极材料。

3 结 论

1）将榴莲壳原料及其经250 ℃、10 h制备的水热焦分别与MgAl-LDH复合，获得复合焦MgAl-LDH@DP与MgAl-LDH@HC，两者相比，MgAl-LDH@HC有更强的活性和亲水性，且对LDH纳米片有较好的分散性。

2）同MgAl-LDH@DP相比，MgAl-LDH@HC呈不规则片状结构，表面疏松多孔，BET比表面积为62.96 m2/g，BJH累积吸附孔容积为0.24 cm3/g，平均孔径14.81 nm，均高于前者，更有利于电荷储存和电子传输。

3）在KOH溶液为电解质、复合焦为工作电极的三电极系统中，循环伏安曲线接近矩形，恒电流充放电曲线接近三角形。同MgAl-LDH@DP相比，MgAl-LDH@HC有较好的电容特性和倍率性能；在低频时，交流阻抗曲线斜率更大，离子扩散阻力相对较小，有潜力作为超级电容器电极材料。

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Durian shell composite biochar prepared by hydrothermal carbonization and its electrochemical properties

Dong Xiangyuan, Zhang Hengrui, Chen Xiang, Li Shuo, Guo Shuqing※

(,,211167,)

Hydrothermal Carbonization (HTC) can widely be used to convert the dry/wet biomass (green and renewable materials) directly into the hydrochar with a rich oxygenated functional group (a high value-added carbonaceous material). There is a promising potential application of hydrochar in energy storage in recent years. Nevertheless, a relatively low capacitance of hydrochar has limited to serve as electrode materials. Recently, Layered Double Hydroxide (LDH) has also been considered as one of the most promising electrode materials, due to the high energy density, dispersed active sites, and cheap raw materials. However, the LDH extension has been confined to a relatively weak electrical conductivity and mechanical stability. Therefore, combing the LDH and hydrochar may be a promising trade-off to develop high-efficient electrode materials. Herein, the hydrochar (HC) was prepared through HTC using durian shell (DP) at 250℃ and 10h. Then magnesium aluminum Layer Double Hydroxides (MgAl-LDH) were decorated on the surface of HC, in order to obtain the MgAl-LDH@HC composite. MgAl-LDH was also decorated on the surface of DP raw materials to explore the effect of HTC process on the performance of the composite. The microstructure of MgAl-LDH@DP and MgAl-LDH@HC were characterized using X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and scanning electron microscopy (SEM). An electrochemical test was also carried out for the properties of the composite. The results show that the cellulose crystal structure of the durian shell was destroyed after HTC treatment, where the carbon content of HC was 70.29%. The XRD pattern of MgAl-LDH@HC presented the sharp peaks at 11.23°, 22.64°, 33.98°, and 60.11° of 2, being assigned to the (003), (006), (012), and (110) planes, respectively, indicating a typical hydrotalcite-like structure. The XRD spectra also illustrated that the MgAl-LDH was successfully decorated on the surface of HC. In MgAl-LDH@DP, there were strong peaks of cellulose crystallinity structure at 22.37° and 34.39°, in spite of the characteristic peaks of LDH in the XRD spectra. There were much stronger active oxygenated functional groups, while much higher dispersion for the LDH nanosheets in the MgAl-LDH@HC, compared with the MgAl-LDH@DP. In MgAl-LDH@HC, a strong polymer characteristic peak at 1622 cm-1contributed to the activity and hydrophilicity of the composite as electrode materials. The XPS spectra of MgAl-LDH@HC presented the strong C 1s, O 1s peaks at 284.80 and 532.14 eV, while the weak Mg 2p, Al 2p peaks at 50.31 and 74.71 eV, respectively. In the C 1s spectra, three peaks centered at 284.53, 285.73, and 288.18 eV corresponding to the C=C, C=O chemical bonding. In the O 1s spectra, three peaks centered at 531.08, 531.93, and 532.78 eV identifying as Al2O3,-OH and -O-, C=O, respectively. These functional groups significantly increased the hydrophilicity, wettability and activity of composite in the electrode solution. SEM images showed that the MgAl-LDH@DP contained a lot of needle-like structures, whereas, the MgAl-LDH@HC presented irregular lamellar structures with porous surfaces. In MgAl-LDH@HC electrochemical test, the Brunauer-Emmett-Teller (BET) surface area was 62.96m2/g, the average pore diameter was 14.81 nm, and the Barrett-Joyner-Halenda (BJH) cumulative pore volume was 0.24 cm3/g, indicating higher properties than those of MgAl-LDH@DP. It inferred that the structure of MgAl-LDH@HC was more conducive to charge storage and electron transmission. Three electrode systems were constructed, with the composite as working electrode and the KOH solution as electrolyte. They were close to rectangle and triangle in the cyclic voltammetry and galvanostatic charge-discharge curve. Higher capacitive property and rate performance were achieved in the MgAl-LDH@HC, compared with the MgAl-LDH@DP. The slope of impedance curve was much larger for the MgAl-LDH@HC at the low frequency, indicating a relatively smaller ion diffusion resistance. Therefore, the MgAl-LDH@HC can be expected to serve as potential electrode materials for supercapacitors.

biochar; hydrochar; electrochemical properties; durian shell; layered double hydroxide

董向元，张恒瑞，陈祥，等. 水热碳化制备榴莲壳复合焦及其电化学性能[J]. 农业工程学报，2021，37(8)：316-322.doi：10.11975/j.issn.1002-6819.2021.08.036 http://www.tcsae.org

Dong Xiangyuan, Zhang Hengrui, Chen Xiang, et al. Durian shell composite biochar prepared by hydrothermal carbonization and its electrochemical properties[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2021, 37(8): 316-322. (in Chinese with English abstract) doi：10.11975/j.issn.1002-6819.2021.08.036 http://www.tcsae.org

2021-01-08

2021-04-07

国家自然科学基金项目（51206194）；南京工程学院引进人才科研启动基金（YKJ201811和YKJ201812）

董向元，博士，副教授，研究方向：生物质水热转化及有效利用。Email：dongxiangyuan@163.com

郭淑青，博士，教授，研究方向：生物质热化学转化及有效利用。Email：shuqing.guo@163.com

10.11975/j.issn.1002-6819.2021.08.036

TK6

A

1002-6819(2021)-08-0316-07